1. Field of the Invention
This invention relates generally to a thermal spray apparatus and method of thermally materials and, in particular, to a thermal spray apparatus with a spray gun capable of coating relatively small bores.
2. Background Art
A particularly useful high pressure plasma coating process is the Plasma Transferred Wire Arc (xe2x80x9cPTWAxe2x80x9d) process. The PTWA process is capable of producing high quality metallic coatings for a variety of applications such as the coating of engine cylinder bores. In the PTWA process, a high pressure plasma is generated in a small region of space at the exit of a plasma torch. A continuously fed metallic wire impinges upon this region where the wire is melted and atomized by the plasma. High speed gas emerging from the plasma torch directs the molten metal towards the surface to be coated. PTWA systems are high pressure plasma systems.
Specifically, the PTWA thermal spray process melts a feedstock material, usually in the form of a metal wire or rod, by using a constricted plasma arc to melt the tip of the wire or rod, removing the molten material with a high-velocity jet of partially ionized gas issuing from a constricting orifice. The ionized gas is a plasma and hence the name of the process. Plasma arcs operate typically at temperatures of 18,000xc2x0-25,000xc2x0 F. (10,000xc2x0-14,000xc2x0 C.). A plasma arc is a gas which has been heated by an electric arc to at least a partially ionized condition, enabling it to conduct an electric current. A plasma exists in any electric arc, but the term plasma arc is associated with plasma generators which utilize a constricted arc. One of the features which distinguishes plasma arc devices from other types of arc generators is that, for a given electrical current and plasma gas flow rate, the arc voltage is significantly higher in the constricted arc device. In addition, a constricted arc device is one which causes all of the gas flow with its added energy to be directed through the constricted orifice resulting in very high exiting gas velocities, generally in the supersonic range. There are two modes of operation of constricted plasma torchesxe2x80x94non-transferred mode and transferred mode. The non-transferred plasma torch has a cathode and an anode in the form of a nozzle. In general, practical considerations make it desirable to keep the plasma arc within the nozzle with the arc terminating on the inner nozzle wall. However, under certain operating conditions, it is possible to cause the arc to extend outside the nozzle bore and then fold back, establishing a terminal point for the arc on the outside face of the anode constricting nozzle. In the transferred arc mode, the plasma arc column extends from the cathode through a constricting nozzle. The plasma arc extends out of the torch and is terminated on a workpiece anode which is electrically spaced and isolated from the plasma torch assembly.
In the plasma transferred wire arc thermal spray process, the plasma arc is constricted by passing it through an orifice downstream of the cathode electrode. As plasma gas passes through the arc, it is heated to a very high temperature, expands and is accelerated as it passes through the constricting orifice often achieving supersonic velocity on exiting the orifice, towards the tip of the wire feedstock. Typically, the different plasma gases used for the plasma transferred wire arc thermal spray process are air, nitrogen, or an admixture of argon and hydrogen. The intensity and velocity of the plasma is determined by several variables including the type of gas, its pressure, the flow pattern, the electric current, the size and shape of the orifice and the distance from the cathode to the wire feedstock.
The prior art plasma transferred wire arc processes operate on direct current from a constant current type power supply. A cathode electrode is connected to the negative terminal of a power supply through a high frequency generator which is employed to initiate an electrical arc between the cathode and a constricting nozzle. The high frequency arc initiating circuit is completed by the momentary closure of a pilot arc relay contact allowing direct current to flow from the positive terminal of power supply through a pilot resistor to the constricting nozzle, through the high frequency arc formed between the cathode and the constricting nozzle, through the high frequency generator to the negative terminal of the power supply. The high frequency circuit is completed through the bypass capacitor. This action heats the plasma gas which flows through the orifice. The orifice directs the heated plasma stream from the cathode electrode towards the tip of the wire feedstock which is connected to the positive terminal of the power supply. The plasma arc attaches to or xe2x80x9ctransfersxe2x80x9d to the wire tip and is thus referred to as a transferred arc. The wire feedstock is advanced forward by means of the wire feed rolls, which are driven by a motor. When the arc melts the tip of the wire, the high-velocity plasma jet impinges on the wire tip and carries away the molten metal, simultaneously atomizing the melted metal into fine particles and accelerating the thus formed molten particles to form a high-velocity spray stream entraining the fine molten particles.
In order to initiate the transferred plasma arc a pilot arc must be established. A pilot arc is an arc between the cathode electrode and the constricting nozzle. This arc is sometimes referred to as a non-transferred arc because it does not transfer or attach to the wire feedstock as compared to the transferred arc which does. A pilot arc provides an electrically conductive path between the cathode electrode within the plasma transferred wire arc torch and the tip of the wire feedstock so that the main plasma transferred arc current can be initiated. The most common technique for starting the pilot arc is to strike a high frequency or a high voltage direct voltage (DC) spark between the cathode electrode and the constricting nozzle. A pilot arc is established across the resulting ionized path generating a plasma plume. When the plasma plume of the pilot arc touches the wire tip, an electrically conductive path from the cathode electrode to the anode wire tip is established. The constricted transferred plasma arc will follow this path to the wire tip.
U.S. Pat. No. 5,808,270 addresses a number of problems in the prior arc related to plasma torch operation. U.S. Pat. No. 5,808,270 is hereby incorporated by reference. Such problems include double arcing, electrical shorting due to metallic dust being attracted to the cathode, and the buildup of coating material on the outer surface of the torch which faces the surface being coated. Furthermore, problems associated with the starting of spraying often cause a xe2x80x9cspitxe2x80x9d or large molten globule to be formed and propelled to the substrate. This globule may cause an imperfection by being included into the coating as the coating builds up on the substrate. However, because of the complexity and the number of individual components of the plasma torch of U.S. Pat. No. 5,808,270, this torch is somewhat limited by how small a bore may be coated. Accordingly, there exists a need for an improved plasma spray torch that can coat smaller diameter bores.
The present invention overcomes the problems encountered in the prior art by providing a plasma transferred wire arc torch assembly that includes a monolithic block assembly combining into a single component several features that have previously been separate components. The monolithic block of the present invention combines the functions of a wire guide, an air baffle and a nozzle. Integration of this components into one component allows for a reduction in size of the plasma transferred wire arc torch assembly. Accordingly, the plasma transferred wire arc torch assembly of the present invention is able to coat the inside of smaller diameter bores than the assemblies of the prior art specifically, the assembly of the present invention is able to coat bores of diameter equal to or greater than about 1.3 inches. Furthermore, the monolithic block assembly of the present invention is simple to fabricate thereby resulting in a reduction in the cost of fabricating a plasma transferred wire arc torch assembly.
In another embodiment of the present invention, a method of coating a surface with a metallic coating utilizing the plasma transferred wire arc torch assembly and the monolithic block assembly of the present invention is provided. The method of the invention comprises initiating and sustaining a plasma in a plasma gun which incorporates the plasma transferred wire arc torch assembly and the monolithic block assembly of the present invention.